By Gregory Mowat, Forensic Tile Consultants
There’s been a dramatic growth of stone tile installations in the last 40 years. As such, the success of any stone tile installation requires the consideration of the stone’s specific properties and the understanding of those properties due to performance. Here’s what you need to know to avoid stone tile installation failures.
Igneous, Metamorphic and Sedimentary
Stones are formed by three different processes, including igneous, metamorphic and sedimentary in the structure of the earth. Igneous stones are formed by cooling of molten or semi molten material. Granite and basalt are igneous and primarily made of quartz and feldspar. They are typically denser, more acid resistant, have a higher resistance to abrasion, and lower absorption compared to a stone tile. Metamorphic stones are formed by undergoing a change in composition resulting from intense heat and pressure. Marble, gneiss, quartzite, serpentine, slate, and soapstone are examples of metamorphic stones. Sedimentary stones are formed by clasts, or particles, that settle and are then cemented together by an agent. Limestone, onyx, sandstone and travertine are sedimentary stones. Agglomerate tiles and other engineered/fabricated tiles include a variety of stones assembled by resins. Their performance relies on the quality and performance of the resin and may be more water sensitive.
Stone management is essential. It includes communication between the parties, designer, seller, and buyer to eliminate stone management failures. Fabricators, importers, and distributors, are experts in our industry and have a duty to state if the product is suitable for intended usage. When stone tiles are selected and approved with known variations, then fabricators, importers, and distributors have a duty to supply what was ordered, or verify that the difference in the product delivered is acceptable to the buyer, designer, contractor and/or end user. Fabricators, importers, and distributors are liable for supplying inferior stone products when the quality or performance is not what was expected.
Considerations for Installers
Water sensitivity is known for serpentine and green-colored marble tile. The stone tiles may be subject to curling or warping when exposed to water during installation. Green and blue limestones, agglomerate and resin-based tiles may warp or curl. Most mortar manufacturers are capable of testing any stone tile to make recommendations for appropriate setting materials. The mortar manufacturer may also know if the stone tile is mildly water sensitive and recommend a mortar with reduced water volume.
Shading and Variation in color, shade and character including, veining throughout any given lot, should be approved prior to installation. Blending of the stone tiles prior to installation or during installation will help in avoiding blocking where one shade is surrounded or bordered by a different shade.
Marble Soundness classifications are A, B, C, and D in descending order of soundness; ascending order of reinforcement and or repair requirements. The C and D stones may have too many veins and inclusions and are not recommended for wet or exterior areas.
Iron Staining, slates in particular, may contain soluble iron that causes bleeding if installed in wet areas. Yellowing of light-colored marbles may be due to iron oxide, especially when installed in wet areas.
Travertine Tiles have holes and voids which are commonly filled at the fabrication site with cementitious or resinous filler. Travertine may be vein cut or fleuri cut, also called crosscut. Different qualities of travertine are available pending the amount of voids and holes, or density of the travertine tiles. High heel shoes can create a concentrated load or point load and may fracture the surface of travertine tiles where voids occur. Also, travertine tiles with voids may support microbial growth in wet area installation.
The use of Fiberglass Mesh-Reinforced Stone Tiles may occur and be applied to the back side of the stone tile when a fabricator is working to reduce breakage during polishing or honing, or to reduce breakage during shipment. Most fabricators do not issue a warning or communicate when this change in production takes place. The fiberglass mesh-reinforcing may be completed with epoxy or polyester resins. As long as the fiberglass mesh does not pull right off the back of the stone tile, then installation can be performed by switching to an epoxy bonding mortar.
Even within the same production of a stone tile, part of the shipment may have the natural stone tile back while another part may have the mesh reinforcement. Upon discovery of a fiberglass mesh-reinforcement backing, a change order may be necessary since a higher cost for installation may be necessary for the epoxy bonding mortar you’ll now have to use. If in doubt, contact your favorite mortar manufacturer for the proper installation mortar and procedure.
Mixing of stone tiles may have a long-term effect on maintenance if the tiles have different abrasion resistance levels. The end user needs to be aware of long-term restoration when resurfacing these differently types of stones. In areas subject to freeze/thaw conditions, do not install stone tiles subject to failure from freeze/thaw conditions. All slates and sandstones are not equal. Use caution when installing slate or sandstone where there is constant wetting and drying occurring. Obviously, avoid using polished stone tiles at building entry points where the stone tile could get wet. Volcanic tuft, also called Canterra and Adoquin, do not perform well in swimming pool surrounds and in vehicular traffic areas. Flagstone does not belong in vehicular traffic areas, especially without adequate movement joints.
Inspection of the substrate the stone tile is to be installed includes verifying no contaminants are on the substrate, the substrate is within acceptable tolerance, and the substrate meets L/720 deflection requirements for stone tile installations. Preplanning is necessary to determine movement joint locations and appropriate setting materials. The Marble Institute of America (MIA) recommends installing a waterproof membrane between stone tiles and concrete slab on-grade installations to reduce moisture migration from underneath affecting the stone tile. For stone tiles known for iron staining, six-sided sealing with a penetrating sealer is beneficial and improves bonding strength to the setting materials.
When a substrate is not within acceptable tolerance, reject the substrate or bring the substrate within acceptable tolerance prior to installation of any sound mat, radiant heating, waterproof membrane meeting ANSI A118.10, or crack isolation membrane meeting ANSI A118.12 requirements prior to proceeding with the installation. Where stone tile is to be directly bonded to a substrate, the maximum allowable variation is 1/8” in 10 feet when measured from the high points in the surface. Where stone is to be installed over a wire-reinforced mortar bed, allowable variation is 1/4” in 10 feet from the required plane. Thinset mortars and medium bed mortars, which are now called large and heavy tile mortar, are not designed nor recommended to be used as a leveling material prior to installation of stone tile assembly.
Setting materials with improved bond strength and deformability are required for above ground structural slabs and other floors subject to movement and/or deflection. If direction is given to bond directly to an already installed waterproof membrane, then verify that the waterproof membrane meets ANSI A118.10 requirements, or reject the waterproof membrane for installation of stone tile in direct bond.
Also, backerboard as a substrate not installed to the backerboard manufacturer’s specific instructions may cause indent fracturing of stone tiles directly above the seams of the backerboard.
Positive slope must be included in design and installation of balconies, courtyards, patios, plaza decks, roofs, exterior walking surfaces and swimming pool decks to prevent ponding water. Ponding water supports bio-organic growth, shaling, spalling and deterioration of stone installations, as well as slippery walking conditions. Perimeter walls should drain water onto the top of the stone tile surface. Flat balconies, landings, and roof decks constructed without a slope often leak over occupiable spaces, destroying property and resulting in decomposition of surfaces and framing members of the building. All surfaces should drain away from the building structures.
Movement (expansion) joints for exterior installations exposed to weather and interior installations exposed to sunlight or moisture (wet areas), the maximum spacing is a 1/2″ movement joint set 12 feet apart and installed where abutting restricting surfaces. For interior installations, maximum spacing is 24 feet apart with a 1/2” movement joint and installed where abutting restricting surfaces. For complete information about movement joint requirements, review the Tile Council of North American Assembly Method EJ171. If as an installer/contractor, you are directed not to install movement joints, then you must confirm in writing to protect your liability with future failures caused by lack of proper movement joints.
Moisture migration from within, under, and from the perimeter of concrete slabs can contribute to stone tile surface deterioration when the stone tile is directly bonded to a concrete slab. Surface deterioration is common with moisture migration in type C and type D marbles. The MIA recommends installing a waterproof membrane between the concrete slab on-grade and the stone tiles. Waterproof membrane must meet the ANSI A118.10 requirements and installed per ANSI A108.13, while following manufacturer’s installation instructions explicitly. Adjacent landscaping should have adequate drainage to accommodate high usage water flow, and not spray water onto stone tile surfaces.
White thin sets are made by mortar manufacturers specifically for installing stone tiles. Lightweight mortars are available in white thin sets. Gray thin sets can bleed through many stone tiles including marble, limestone, onyx and granite. Epoxy may be recommended for the installation of moisture-sensitive stone tiles and/or rapid-setting mortar. If in doubt, consult with the mortar manufacturer. Mortars installed in swimming pools, spas, fountains, and steam rooms may require a longer cure time before water is introduced. See the mortar manufacturer’s requirements for curing.
Bond adhesion should achieve at least 95% of stone tiles with all edges of the stone tile fully supported. Minimum grout joint width is 1/16“. Avoid designs and installation that require tightly butted joints (no joint). Grout joints 1/8” or smaller use non-sanded grout. Grouts manufactured by mortar manufacturers have greatly improved with advanced technology. Follow installation instructions by the grout manufacturer and ANSI A108.10 requirements.
Spot setting with mortar is not a recommended practice for installing stone tiles.
Always refer to installation standards that are updated yearly for any new findings and requirements. These guides include the annual Tile Council of North America Handbook for Ceramic, Glass, and Stone Tile Installation. Since 2011, the TCNA Handbook has included specific installation assemblies for stone tile; The American National Standard Specifications for the Installation of Ceramic Tile ANSI A108, A118 and A136; The Marble Institute of America Design Manual.